This invention relates generally to optical gain devices and, more specifically, to multiple wavelength and broadband sources of optical energy.
An optical amplifier is a device that increases the amplitude of an input optical signal fed thereto. If the optical signal at the input to such an amplifier is monochromatic, the output will also be monochromatic, with the same frequency. Fiber-based optical amplifiers are an example of devices that require a predetermined optical energy input. One form of optical amplifier known in the art is based on stimulated Raman scattering (SRS), and is referred to generally as a xe2x80x9cRaman amplifier.xe2x80x9d In a Raman amplifier, optical pumping energy is injected into an optical fiber medium through which the signal to be amplified is conducted. The optical pumping energy, via SRS, allows for a transfer of optical power to a wavelength longer than the pumping wavelength, due to the excitation of a vibrational mode in the medium that provides gain at the longer wavelength.
In optical communication media, it is desirable to provide a wide bandwidth to accommodate many channels on the same transmission link. Intrinsic to maintaining such a wide bandwidth is the ability to control signal degradation such that sufficient signal power is maintained in each of the channels. Since optical amplifiers are often used to amplify transmitted signals, such amplification should therefore allow the signal power in each of the channels to be maintained at a relatively equal level. In prior art systems using Raman amplifiers, the effective bandwidth of a system is therefore often limited by the bandwidth of the amplifier itself. Increase of the amplifier bandwidth would allow for an increase in the bandwidth of the communication system, and a corresponding increase in the number of channels available.
In accordance with the present invention, optical sources and pumping strategies for multiple wavelength or broadband applications. Applications of such sources include the pumping of Raman amplifiers. Such multiple wavelength pumping can be used to modify the gain spectrum of an amplifier and enhance its functionality for a desired application, such as wideband amplification of communication signals.
In one embodiment of the invention, an optical signal generator includes an optical pump source that provides pump energy having a predetermined wavelength range. The optical pump source is coupled into an optical gain medium that responds by producing optical signal energy within an output wavelength range. In a preferred embodiment, the gain medium provides gain via stimulated Raman scattering. Multiple output couplers are provided, each of which couples optical energy in a different wavelength range out of the gain medium. In the preferred embodiment, the output couplers are partially reflective gratings that each has a different wavelength selectivity. The partially reflective gratings are paired with other reflectors to establish multiple wavelength resonance conditions through the gain medium. This results in an output having multiple distinct wavelength peaks.
In a variation of this embodiment, the output couplers are part of a cascaded Raman resonator (CRR) type configuration. The CRR uses sets of gratings that are wavelength selective and that progressively shift resonant wavelengths within the gain medium through several Stokes orders. The output couplers are therefore typically reflective at a wavelength that corresponds to the highest of the Stokes orders of the system. By using multiple output couplers within the wavelength range of the highest Stokes order, an output is generated that has multiple wavelength peaks. In one variation of this embodiment, the output coupler gratings are located in separate parallel optical paths, along with their wavelength matched counterparts. The optical signal from the gain medium corresponding to the next lowest Stokes order is coupled into the parallel paths with a wavelength selective optical coupler. In each of the parallel paths is located a secondary gain medium, such that Raman amplification at the wavelength corresponding to the highest Stokes order occurs in each of the parallel paths. The differently tuned output couplers thus select the different output wavelengths in each of the parallel paths. These output wavelengths may then be combined using another wavelength selective coupler, such as a wavelength division multiplexer (WDM). Alternatively, only some (or none) of the outputs may be combined, allowing them to be delivered to different pumping applications, or to different stages of a multiple stage amplifier.
In a variation of the CRR embodiment, one or more of the CRRs may include a wavelength tunable reflector. The wavelength tunable reflector may be adjusted to reduce the efficiency of one of the resonant cavities established by one of the CRRs, and thereby lower the output power of that CRR. Since the CRRs compete for gain within the gain medium, the power of the other CRRs will correspondingly increase. This allows the relative output power at the different output wavelengths to be adjusted. If two tunable reflectors are used with more than one of the CRRs, the relative output power between the CRRs may be adjusted over a wider range. Although the preferred embodiment makes use of two CRRs, more than two may also be used.
In another embodiment of the invention, overlapping resonant cavities are used for which separate output couplers are used for each cavity. A shared primary gain medium is located in a common optical path, and each cavity also has its own separate gain medium, through which optical energy in the other cavity does not pass. In one version of this embodiment, portions of the separate cavities are located in separate optical paths. Wavelength selective couplers are used to couple the different wavelengths corresponding to the different resonant cavities between the separate optical paths. Output couplers for the different wavelengths are located in the separate optical paths, and are preferably partially reflective gratings. Highly reflective gratings for the output wavelengths are located in the common optical path or, in the preferred embodiment, a single highly reflective grating is used in the common optical path that is reflective at the different wavelengths, such as a broadband grating that encompasses the wavelength range of each of the output wavelengths. In an alternative embodiment, the reflectors used in the parallel paths are highly reflective, and the grating or gratings used in the common optical path are partially reflective. Thus, the outputs are coupled out of the common optical path, and a wavelength selective coupler may be used to separate the output wavelengths from other optical energy in the common optical path.
In yet another embodiment of the invention, a gain medium is used that has a known gain profile when pumped by a particular pump source. Along with the resonator reflectors is provided a filter located within the resonant cavity established by the reflectors, that selectively attenuates wavelengths within the gain spectrum. In the preferred embodiment, the attenuation is matched to known variations in the gain profile such that gain maxima are reduced and the overall gain profile is flattened. In an extension of this embodiment, the gain medium is pumped and multiple output couplers are used to output optical energy in multiple output wavelength ranges. Because of the gain flattening provided by the filter, the gain produced in each of the output wavelength ranges is approximately the same.
In still another embodiment of the invention, a source is provided with a CRR configuration, including multiple reflector pairs at wavelengths corresponding to the different Stokes orders of the CRR. The source also includes a gain medium and resonant cavity reflectors, including multiple output couplers each of which selects and outputs a different wavelength within an output wavelength band. In this embodiment, however, the reflectors at the highest of the Stokes orders, which preferably include a partially reflective output coupler reflector, have a broad wavelength selection band, such that the optical energy output from the source extends across a relatively wide bandwidth range. Thus the source is advantageous for applications such as the broadband pumping of a Raman amplifier.
In a different embodiment of the invention, a multiple output wavelength source is provided that uses polarization maintaining (PM) fiber as a Raman gain medium. Cavity reflectors are integrated into the PM fiber, one of which is preferably a partially reflective output coupler. The PM fiber gain medium is then pumped, and two resonant conditions develop, respectively, at two orthogonal polarizations within the fiber. Each of these resonant signals develops at a different wavelength due to the birefringence of the fiber, and by coupling them out of the gain medium, a multiple wavelength output is provided. Preferably, the PM fiber has a relatively high birefringence.
In another embodiment of the invention, broadband pumping for a Raman amplifier is provided by the use of a laser source operating in coherence collapse. The source uses an optical gain element, such as a semiconductor gain chip, a Fabry-Perot diode or a flare diode. The output from the gain element is coupled into a transmission medium, such as an optical fiber, along which is positioned a low level reflector. In the preferred embodiment, the low level reflector is a grating written into the transmission fiber, and has a reflectivity of less than 10%. Moreover, the grating is positioned a relatively large distance from a reflector adjacent the gain medium, e.g., greater than 20 cm, and ensures broadband operation of the source.
A multiple wavelength pumping arrangement is also provided herein for multiple separate optical paths, such as the separate transmission fibers of an optical fiber cable. In accordance with this embodiment, multiple pump sources are used, each having a different output wavelength. Outputs from the different pump sources are combined into each of the different fibers, such that each fiber acquires pumping energy from each of the sources. In this way, broadband pumping energy is provided equally to each of a number of related amplification media.